The immunodeficiency associated with advanced cancer is well established. One aspect of immunity deranged in cancer patients is T-cell function. The Thymus gland plays a significant role in the maturation of the T-lymphocyte which, in turn, is the principal cell participating in cell mediated immunity (CMI). Of the various Thymic hormones extracted from the Thymus or the serum, 3 (Thymosin, Thymosin alpha 1, and THF) and possibly by a fourth (Thymopoietin) are available to us for in vitro study, with a view to clinical use in the future. We have demonstrated the effects of in vitro thymosin and/or % E-rosette forming cells (E-RFC), lymphocyte blastogenic response (LBR), in vivo graft-versus-host reaction (GVHR), suppressor cell activity in cancer patients, and natural killer (NK) activity in normals. We propose to compare, in vitro, the restorative capacity of the various thymic hormones on conventional assays such as % E-RFC, LBR, and intracellular cAMP, as well as more sophisticated assays. The latter will attempt to determine the effect of thymic hormones on suppressor cell activity in cancer patients, on NK activity, on the patients' lymphocytes injected into an immunosuppressed rat, with the subsequent development of a GVH reaction; and on the ability of patients' lymphocytes to release interferon into the culture medium following incubation with thymic hormones. We assume that a patient, who responds favorably to in vivo thymic hormone treatment can be identified by in vitro studies. Moreover, we hope to predict the exact hormones to which a patient's deranged CMI would respond better. Since the various thymic hormones effect T-cells at a different stage of maturation, it is feasible that a patient could respond to sequential use of more than one hormone, i.e. one affecting T0 and one affecting T1, thereby inducing the emergence of the mature T2. The study is designed as a prelude to the introduction of the thymic hormones to clinical use, when these assays will be monitored following in vivo thymic hormone treatment.